1. Industrial Field of the Invention
The present invention relates to a reciprocating piston fluid pump installed in a vehicle and driven by an engine of the vehicle to fulfill a function as a vacuum pump or the like.
2. Description of Related Art
As a representative of such a pump, a vacuum pump is used for a brake booster of a diesel engine.
This vacuum pump includes a pump upper chamber and a pump lower chamber defined above and below an actuating piston in the pump chamber. A seal member fitted in an annular groove formed on an outer periphery of the piston acts as a slidable portion between the piston and a cylinder. A rod of the piston is driven by a cam which is moved by the engine to thereby reciprocatingly move the actuating Distort vertically, so that the pump is driven by the vehicle engine.
The piston is formed as a disk plate having a thickness enough to retain a seal member which is fitted in a groove formed on the outer periphery. The thickness of the piston is thus uniform in the radial direction from its outer periphery to its central portion, so that the weight of the piston is inevitably large. For these reasons, a compression spring which can exert a large urging force is required. The compression spring is provided on the piston rod to press the rod against the cam, so that the piston rod can be operated following the rotation of the cam.
The rod is designed to vertically slide within a rod seal. A small amount of oil mist is sucked into the pump lower chamber through a minute gap between the rod and the rod seal, the gap being opened to the interior of the engine.
In the above-described conventional pump, only a small amount of oil mist is sucked into the pump lower chamber through the minute gap between the rod and the rod seal. For these reasons, when the pump is driven in a high- temperature atmosphere and the air introduced in the pump chamber is evacuated, the small amount of oil stored in the pump chamber is taken away during the evacuation. As a result, the slidable portion with respect to the piston cannot be sufficiently lubricated, so that wear of the piston ring and seizure of the cylinder occur.
Due to surface roughness of the sliding surfaces of the piston ring and the cylinder and the wear of the piston ring, the sealing ability at the piston slidable portion is deteriorated, which results in deterioration of a pumping performance.
Further, an amount of generation of heat is increased because of the surface roughness of the sliding surface, so that a volume of an O-ring is expanded. Because contact pressure between surfaces of the cylinder and the O-ring is increased due to the volume expansion of the O-ring, the resistance between the cylinder and the O-ring is increased more when they slide against each other. Such vicious cycle may be caused in the conventional pump.
Because the piston slidable portion is unusually abraded due to the increase of the sliding resistance between the piston and the cylinder and because the load of the compression spring is large, force applied to contact portions between the rod and the cam is increased so that the contact portions between the rod and the cam are abnormally abraded.